Rotating electrical machines, such as motors and generators, generally comprise a rotor and a stator which are arranged such that a magnetic flux is developed between these two. In a permanent magnet “PM” electrical machine, a number of permanent magnets are usually mounted on the rotor, while the stator is provided with stator windings. The permanent magnets and electrical currents in the stator windings cause a magnetic flux to flow across the air-gap between the rotor and the stator.
A rotor of a permanent magnet electrical machine comprises typically a ferromagnetic core structure, a shaft, and permanent magnets arranged to produce magnetic fluxes so as to form magnetic poles of the rotor. When designing a rotor of a permanent magnet electrical machine there are various aspects to be taken into account. The electro-magnetic characteristics of the rotor should be such that it can provide desired performance with sufficiently low losses, the mechanical characteristics of the rotor should be such that the mechanical structures of the rotor can withstand the ensued mechanical stresses, and the thermal characteristics of the rotor should be such that the heat caused by the losses can be removed from the rotor. Furthermore, the high price of high-quality permanent magnet material causes a need to minimize the amount of permanent magnet material used in the rotor.